Spinal implants such as interbody fusion devices are used to treat degenerative disc disease and other damages or defects in the spinal disc between adjacent vertebrae. The disc may be herniated or suffering from a variety of degenerative conditions, such that the anatomical function of the spinal disc is disrupted. Most prevalent surgical treatment for these conditions is to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for a portion of the annulus, by way of a discectomy procedure. A spinal fusion device is then introduced into the intradiscal space and suitable bone graft or bone substitute material is placed substantially in and/or adjacent the device in order to promote fusion between two adjacent vertebrae.
Certain spinal devices for achieving fusion are also expandable so as to correct disc height between the adjacent vertebrae. Examples of expandable interbody fusion devices are described in U.S. Pat. No. 6,595,998 entitled “Tissue Distraction Device”, which issued on Jul. 22, 2003 (the '998 patent), U.S. Pat. No. 7,931,688 entitled “Expandable Interbody Fusion Device”, which issued on Apr. 26, 2011 (the '688 patent), and U.S. Pat. No. 7,967,867 entitled “Expandable Interbody Fusion Device”, which issued on Jun. 28, 2011 (the '867 patent). The '998 patent, the '688 patent and the '867 patent each discloses sequentially introducing in situ a series of elongate inserts referred to as wafers in a percutaneous approach to incrementally distract opposing vertebral bodies to stabilize the spine and correct spinal height, the wafers including features that allow adjacent wafers to interlock in multiple degrees of freedom. The '998 patent, the '688 patent and the '867 patent are assigned to the same assignee as the present invention, the disclosures of these patents being incorporated herein by reference in their entirety.
Certain interbody fusion devices also include hollow portions or chambers that are filled with suitable material such as bone graft to promote fusion between vertebral bodies. The extent and size of the chambers establish areas of contact that are configured so as to assure maximum contact between the bone graft and the vertebral bodies. Sufficient surface area of the device surrounding the chambers needs to be maintained in order to provide an appropriate load bearing surface to withstand the compressive forces exerted by the opposing vertebral bodies. In addition, where expandable interbody fusion devices are used to correct height within the intradiscal space, the effect of shear forces on the expanded device due to torsional movement of the spine also needs to be considered.
Accordingly, there is a need to develop expandable interbody fusion devices with bone graft chambers that take into account and balance these factors.